Metrology structure and methods

ABSTRACT

A method of indicating the progress of a sacrificial material removal process, the method, comprising; freeing a portion of a member, the member being disposed in a cage and laterally surrounded by the sacrificial material; and preventing the freed portion of the member from floating away by retaining the freed member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/832,367 filed on Apr. 27, 2004, now U.S. Pat. No. 7,365,405, which isincorporated by reference herein in its entirety.

BACKGROUND

Sacrificial materials are often used in the fabrication of devices, suchas MEMS (microelectromechanical system) devices. These sacrificialmaterials are removed in a later stage in a process flow to generatedesigned empty spaces below or around the devices. This removal processis also commonly called a release process, because the movable parts ofa device are released and free to move in at least one dimension afterthe sacrificial material is removed. The sacrificial material is oftenremoved using chemical processes, such as etching, near the end of waferfabrication. Since the sacrificial material often occupies spaceunderneath the movable devices, it is frequently difficult to determinewhen the removal process is complete using standard optical inspectionof the wafer.

In the past, it has been difficult to accurately monitor or evaluate thecompleteness of the release process. In some processes, devicestructures are scratched or otherwise physically removed to reveal theprogress of the release process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric diagram illustrating a metrology structureaccording to one embodiment of the present invention.

FIG. 2 shows a third layer of a cage according to one embodiment of thepresent invention.

FIG. 3 shows a second layer of a cage according to one embodiment of thepresent invention.

FIG. 4 shows a first layer of a cage according to one embodiment of thepresent invention.

FIG. 5 is a cross-section of a metrology structure before removal of asacrificial material according to one embodiment of the presentinvention.

FIG. 6 is a cross-section of a metrology structure after removal of asacrificial material according to one embodiment of the presentinvention.

FIG. 7 is a cross-section of a plurality of cages and members disposedon a substrate before removal of a sacrificial material according toanother embodiment of the present invention.

FIG. 8 is a cross-section of a plurality of cages and members disposedon a substrate during removal of a sacrificial material according toanother embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIGS. 1, 2, 3, 4, 5, 6, 7, and 8 are provided for illustration purposesonly and are not intended to limit the present invention; one skilled inthe art would recognize various modifications and alternatives, all ofwhich are considered to be a part of the present invention.

Referring to FIG. 1, one embodiment of a metrology structure 10 isshown. In the embodiment, the metrology structure 10 is disposed on asubstrate 12. In some embodiments a MEMS device may be formed on thesubstrate 12. The metrology structure 10 comprises a cage 13 havingsacrificial material (not shown) disposed therein. The cage 13 comprisesa first layer 14, a second layer 16, and a third layer 18, disposedsuccessively on the substrate 12. However, the number of layers are notlimiting on the invention, and any other suitable number of layers canalso be used. Typically, the substrate 12 comprises real MEMS devices(not shown) along with the metrology structure 10. For the purposes ofillustration, FIG. 1 comprises only the metrology structure 10. Forexample, the metrology structure 10 may cover an area of approximately1% on the substrate 12, with the real MEMS device covering approximately99% on the substrate 12. These percentages may, of course, vary. In oneembodiment, the substrate 12 used can be a silicon wafer or glass, forexample. When the fabrication of the MEMS device is finished, themetrology structure 10 can be left on the substrate 12 or diced awayduring a chip dicing process.

Referring to FIG. 2, in one embodiment, the third layer 18 of themetrology structure 10 comprises a third portion 22 of a member disposedin a third portion 24 of the cage 13. In one embodiment, the thirdportion 22 of the member comprises a post 23 (shown in FIGS. 5 and 6)and the third portion 24 of the cage 13 comprises structures such astabs 26 and posts 28. According to one embodiment, the third portion 24of the cage of the layer 18 comprises four tabs 26 and four posts 28.During the fabrication of the metrology structure 10, prior to thedeposition of the third layer 18, a sacrificial material (not shown) ispatterned with holes (corresponding to the post 23 and the posts 28) insuch a way that when the third layer 18 is deposited, the post 23 andthe posts 28 are formed. The post 23 and the posts 28 act as supportcolumns and are used to provide structural support for the third layer18. In some example embodiments, of the metrology structure 10, theheight dimension for the post 23 and the posts 28 can be in the range ofapproximately 0.1 μm to 5 μm. According to one embodiment of the presentinvention, the height dimension for the post 23 and the posts 28 can bein the range of approximately 1 nm to 100 nm. This height dimension mayvary.

Referring to FIG. 3, in one embodiment, the second layer 16 of themetrology structure 10 comprises a second portion 32 of the member andis connected to the third portion 22 of the member of the third layer 18by the post 23. The second layer 16 further comprises a second portion34 of the cage 13 that supports the third portion 24 of the cage 13 ofthe layer 18. The second portion 34 of the cage 13 comprises posts 36that are used to provide structural height therefore and are formed asmentioned above for the layer 18. According to one embodiment, thesecond portion 34 of the cage 13 comprises twelve posts 36. The numberand height of the posts may vary.

Referring to FIG. 4, in one embodiment, the first layer 14 of themetrology structure 10 comprises a first portion 42 of the cage 13 thatsupports the second portion 34 of the cage 13, and a member 44. In oneembodiment, the first layer 14 is designed such that the member 44 isdisposed on only a portion of the substrate 12. According to oneembodiment, the member 44 is disposed in one corner of portion 42 of thecage 13 on the substrate 12. However, the layer 14 can be also bedesigned such that the member 44 can be disposed on other portions ofthe substrate 12. The purpose of the member 44, in some embodiments, isto tilt the member 22, 32 at an angle when the member 22, 32, tips orfalls on to the substrate 12. Thus the tilted member 22, 32 (FIG. 6) canbe more readily discernable to an observer as freed than if the memberwas disposed flat on the substrate 12.

FIG. 5 shows a cross-section of the metrology structure 10, beforeremoval of a sacrificial material according to one embodiment of theinvention. The sacrificial material is shown by the hatched area. FIG. 6shows a cross-section of the metrology structure 10, after removal of aportion of the sacrificial material according to one embodiment of theinvention. Arrows 46 and 48 in FIGS. 5 and 6 represent incident andreflected light, respectively on the top surface of the third portion 22of the member to be freed of the third layer 18 of the metrologystructure 10. Referring to FIGS. 2, 3, 4, 5, and 6, as the third portion22 is freed from the sacrificial material underneath the layer 18 andthe second portion 32 is freed from the sacrificial material underneathand surrounding the layer 16, simultaneously, the freed member will beretained in the cage comprising the third portion 24, the second portion34 and the first portion 42, but will be able to move and drop downwithin the cage. It should be noted that the posts 28 of the layer 18and the posts 36 of the layer 16 not only provide support for the layers18 and 16, respectively, they also prevent the freed member 22, 32 frommoving far in a horizontal direction. Also, during the sacrificialmaterial removal process, when external forces try to pull the freedmember 22, 32 to float up, the tabs 26 of the third portion 24 of thecage 13 block the second portion 32 of the freed member which, in turn,is connected to the third portion 22 of the freed member of the layer18, and hence the possibility of the freed member 22, 32 beingpulled/floated up in a vertical direction and potentially damaging otherdevices on a wafer is prevented.

FIGS. 1, 2, 3, 4, 5, 6, 7, and 8 are not drawn to scale. They provideschematic illustration of certain example embodiments.

In one embodiment of the invention, the cage 13 and members 22, 32, and44 can be made of aluminum or an alloy of aluminum such asaluminum/titanium. However, the above materials are not limiting on theinvention, and any other combination of metals, dielectrics, orpolymeric materials can also be used. The sacrificial material used canbe an organic polymer such as photoresist, or any other suitablematerial such as silicon, or silicon dioxide, for example. The cage 13and members 22, 32 may comprise, in one embodiment, a model structure ofa real MEMS device(s).

According to another embodiment of the invention, a plurality of cagesand members of different sizes in area laterally surrounded by asacrificial material may be disposed on the substrate 12 as shown inFIGS. 7 and 8. For the purposes of illustration, FIGS. 7 and 8 compriseonly three cages and members. The number of cages and members may vary.FIG. 7 shows a cross-section of the cages and members before removal ofa sacrificial material and FIG. 8 shows a cross-section of the cages andmembers during removal of a sacrificial material. The sacrificialmaterial is shown by the hatched area in FIGS. 7 and 8. During thesacrificial material removal process, a smallest area member may befreed first. As the removal process progresses in time, successivelylarger area members will become free. This is because it takes longertime for an etching material to go underneath the larger area member ascompared to the smaller area member during the removal process.Referring to FIG. 8, during the sacrificial material removal process,the members 110 and 120 have been freed whereas the member 130 still hassome sacrificial material left. By using a plurality of cages andmembers of increasing size in area, the progress of the removal processcan be determined. It should be noted that the increase in size is donelaterally as shown in FIGS. 7 and 8 in order to comply with thefabrication techniques. In one embodiment the size of the cages and themembers may comprise a range of 1 μm to 500 μm. In an alternateembodiment, the size of the cages and the members may comprise a rangeof 10 nm to 1000 nm.

According to one embodiment, an observer can determine the state of theremoval process, by detecting a change in the characteristic of thefreed member 22, 32 such as detecting a change in light reflection(shown by the arrow 48 in the FIG. 6) from a top surface of the firstportion 22 of the freed member as compared to the light reflection shownby the arrow 48 in FIG. 5. In an alternate embodiment, the state of theremoval process may be determined by detecting a change in thecharacteristic of the freed member 22, 32 by detecting a change inreflection or scattering of electrons, ions, atoms, or photons.

The metrology structure 10 can be built using standard micro-electronicfabrication techniques such as photolithography, vapor deposition andetching. However, the above techniques are not limiting on the inventionand any other suitable techniques can also be used.

It should be noted that the metrology structure 10 provides an easierevaluation to determine the state of removal processes when using manualor automated visual inspection. The metrology structure 10 can be easilyscaled or otherwise modified to be useful for monitoring the removalprocesses for a wide variety of MEMS designs. The metrology structurecan be used for various standard mask levels and fabrication stepstypically used in fabricating the real MEMS devices, and hence, pursuantto some embodiments, no additional process steps are required to createthe metrology structure.

The foregoing description of various embodiments of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the invention. Theembodiments were chosen and described in order to explain the principlesof the invention and its practical application to enable one skilled inthe art to utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated.

1. A method of indicating the progress of a sacrificial material removalprocess, the method comprising: freeing a portion of a member, themember being disposed in a cage and laterally surrounded by thesacrificial material, wherein the sacrificial material is patterned withholes; preventing the freed portion of the member from floating up inthe vertical direction by retaining the freed member; and detecting achange in a characteristic of the freed portion of the member, whereinthe detecting the change step further comprises detecting a change inreflection or scattering of electrons, ions, atoms, or photons from asurface of the freed portion of the member.
 2. The method as defined inclaim 1, further comprising causing the freed portion of the member totip so that a surface thereon is tilted.
 3. A method of indicating theprogress of a sacrificial material removal process, the methodcomprising; freeing a portion of a member, the member being disposed ina cage and laterally surrounded by the sacrificial material, wherein thesacrificial material is patterned with holes; preventing the freedportion of the member from floating up in the vertical direction byretaining the freed member; and detecting a change in a characteristicof the freed portion of the member by detecting a change in an angle oflight reflected from a surface of the freed portion of the member afterremoval of a portion of the sacrificial material as compared to an angleof light reflected from the surface of the freed portion of the memberbefore removal of the sacrificial material.
 4. A method of indicatingthe progress of a sacrificial material removal process, the methodcomprising: providing a plurality of cages on a substrate in a firstdirection; providing a member in each of the plurality of cages, whereinthe member is laterally surrounded by sacrificial material and thesacrificial material is patterned with holes, wherein the plurality ofcages and members disposed therein increase in area in the firstdirection; and freeing a smallest member within one of the plurality ofcages and successively freeing larger members in the first direction asa removal process progresses in time.